Axially adjustable thrust bearing for jet propulsion units

ABSTRACT

The present invention relates to a jet propulsion unit ( 1 ) for propulsion of a waterborne craft comprising at least one impeller pump ( 3 ) with a pump housing ( 4 ) in which an impeller shaft ( 22 ) or ajournal ( 21 ) connected to said impeller shaft ( 22 ) is is rotatably journalled in bearings, and an impeller ( 7 ) blades ( 24 ), the impeller ( 7 ) being fixed to said impeller shaft ( 22 ) or journal ( 21 ) so as to follow the rotation of the impeller shaft ( 22 ), the impeller blades ( 24 ) being arranged at the impeller ( ) with a play of the blades ( 25 ) between each impeller blade ( 24 ) and the inside ( 15 ) of the pump housing ( 4 ). According to the invention the jet propulsion unit ( 1 ) also comprises an axially adjustable thrust bearing ( 27 ) arranged to cooperate with the impeller shaft ( 22 ) for adjusting said play ( 25 ).

TECHNICAL FIELD

The present invention relates to a jet propulsion unit for propulsion ofa waterborne craft comprising at least one impeller pump with a pumphousing in which an impeller shaft or a journal connected to saidimpeller shaft is rotatably journalled in bearings, and an impeller,having impeller blades, the impeller being fixed to said impeller shaftor journal so as to follow the rotation of the impeller shaft, theimpeller blades being arranged at the impeller with a play between eachimpeller blade and the inside of the pump housing.

STATE OF THE ART AND PROBLEM

Jet propulsion units, also named water jet- or impeller units, of theaforedescribed kind are known in many different designs and dimensions,from smaller units of about 500 kW up to large units of about 30 000 kWand more. Compared to a conventional propeller, a water jet unit has amethod of propulsion that is to a great extent different. Instead of theconventional propeller it has a water pump or impeller pump, as it iscalled, mounted at the hull of the craft. The impeller pump comprises animpeller mounted within an enclosing, an impeller chamber, that is apart of a seawater flow tube at the same time. The impeller is attachedto the end of a drive shaft, the impeller shaft, which is either theengine shaft of the craft or indirectly connected to said shaft. Thepurpose of the impeller is to pressure the seawater creating a veryforceful water jet, from that the name jet propulsion unit.

The force of reaction that results from the water leaving the pump isused to propel the craft. The direction of rotation of the impeller isnever changed but instead a separate “reversing device” is used fordeflecting the direction of the water jet and thereby also the directionof the force of reaction, which then also changes the direction of thepropulsion of the ship. A water jet unit has many advantages compared toa conventional propeller since an impeller pump has an efficiency of upto 90% and more at speeds over 25 knots. This results in higher speed ofthe craft at the same propulsion force or markedly lower fuelconsumption at constant speed and lower propulsion force.

However, today there is still a big gap between on one hand fast butexpensive airfreight and on the other hand cheaper but slower containerfreight by ship. Therefore, there is a desire to be able to build verylarge and very fast ocean-going ships for high-speed transports betweenfor example USA and Europe and such ships would be equipped with verylarge water jet units. Such ships are estimated to reach average speedof 35 knots even at waves at the height of 7.5 meter and the presenttime of transport is estimated to be shortened by a third. To be able tobring about above mentioned ships, water jet units are needed that canreach at least 50 000 kW and that will have a diameter of intake ofabout 3.25 meter compared to the presently known largest diameter of 2.0meter. The planned ships have units with a flow equivalent to about 500m³ per second. It will be understood that enormous units are required tomanage this capacity.

However, larger installations need a higher accuracy at the choice ofthe correct dimension of the water jet, both for achieving an optimisedefficiency and for improvement of the fuel economy. Other common demandson the propulsion unit may be for example good manoeuvrability and alsothe ability to handle tough operation situations such as high sea.Several cooperating factors will decide how well the ship will adapt tothe demands made on it.

By controlling the play between the impeller blades and the impellerchamber the risk that one runs of them contacting each other will besubstantially reduced during normal operation, thus resulting in that ahigher efficiency of the impeller may be obtained. Thus, the playbetween the impeller and the impeller chamber is of great importance tothe propulsion output and since a bigger play reduces the efficiency ofthe impeller pump it is essential that the play is as small as possible.At a planned impeller pump the impeller blades will have a diameter ofabout 4.5 meters and the planned play between the impeller blades andthe impeller chamber will amount to only about 4 millimetres. It isunderstood that e.g. temperature variations, entails problems that makeit difficult to further reduce the play without the hazard ofbreakdowns. The damages that might be the result of the impellercontacting the enclosing are a serious problem. Such contact may be dueto e.g. the vibrations or oscillations caused by cavitation. If, forthis reason, the play instead is doubled, that is from about 0.1% of thediameter to 0.2%, the propulsion output will be reduced by about 1%which involves a not negligible increase in costs.

Because of the gains, calculated as a percentage, that can be obtainedfor each millimetre closer play that can be achieved, it is thus ofutmost importance that the play normally can be kept with as narrowmargins as possible. Of course, at the same time, the demand on themaximally admissible tolerance between the impeller and the enclosingwill be very highly put forward. Furthermore, it is also true that thelarger jet propulsion unit, the larger the demands on the tolerancebetween different parts will be made. If calculated as a percentage, themachine tolerances of an impeller are higher than those of a clockwork.For example, impeller blades with a diameter of up to 4500 millimetresare being turned and grinded with a precision of a few tenths of amillimetre. At speed of for example 40 knots, these small tolerancesoptimise the transformation of shaft-force into driving-force,prolonging the endurance and giving minimal vibrations and sound-levels.

However, one runs the risk that the oscillations and vibrations at someoccasions, for example, when strange objects pass through the pump, willcause such a radial movement of the blades of the impeller that theywill come in contact with the impeller chamber. The risk of damages thatone runs will increase strongly at tough weather conditions, forexample, at high sea one runs the risk of large acceleration or airsuction through the pump. Also, the position of the impeller can bealtered because of movements in the hull of the ship itself. Theconsequences of a breakdown of the jet propulsion unit during wholegale, in the middle of the Atlantic Ocean, could be serious.

Thus, it might be necessary to accept a somewhat reduced efficiency byincreasing the play a little, at certain special situations. Since theimpeller chamber and the impeller blades have a conical interface theplay can be altered if the impeller is moved axially. Thus, it is anessential demand that, when necessary, the play between the impeller andthe enclosing can be adjusted by axial movement of the impeller.

THE OBJECT AND CHARACTERISTICS OF THE INVENTION

It is an object of the present invention to accomplish such a jetpropulsion unit for propulsion of ships, that makes it possible tosubstantially reduce or fully eliminate the risk of the blades of theimpeller contacting the inner wall of the impeller chamber and to beable to make use of the favourable qualities of the jet propulsion unitin a better way than before and also at considerable larger shipdimensions than at the present conventional dimensions of ships. It issuggested, as a solution of the complex of problems, that the thrustbearings included are movably arranged. The movement is suitably done byusing some sort of hydraulic piston arrangement, which also wouldcontribute with a certain dampening function.

The jet propulsion unit according to the invention is characterized inthat the jet propulsion unit also comprises an axially adjustable thrustbearing arranged to cooperate with the impeller shaft for adjusting saidplay.

According to further aspects of the jet propulsion unit according to theinvention:

-   -   the thrust bearing comprises a bearing housing surrounding and        encasing a bearing disc connected to or cooperating with the        impeller shaft and said bearing housing being arranged axially        movable by means of adjusting devices a distance sufficient for        the play between the impeller blades and the inside of the pump        housing to stay within the limits of a maximally and minimally        admissible play of the blades.    -   rods are attached at a distance from and parallel with the shaft        extending through the bearing housing and that the bearing        housing is axially movable along the rods by means of guides.    -   the thrust bearing comprises a bearing housing fixed relative to        the hull of the ship and surrounding and encasing a bearing disc        connected to or cooperating with the impeller shaft the bearing        disc being arranged to be axially adjustable by means of        adjusting devices a distance sufficient for the play between the        impeller blades and the inside of the pump housing to stay        within the limits of a maximally and minimally admissible play        of the blades.    -   the adjusting devices are mounted inside the bearing housing.    -   the adjusting devices comprise at least one hydraulic or        pneumatic cylinder.    -   alternatively, some of or all the adjusting devices may be        mechanical adjusting devices.    -   the bearing disc is a radially protruding part of the impeller        shaft, the journal, or an intermediate shaft especially adapted        for the thrust bearing and connected to the impeller shaft.    -   the bearing housing comprises one or several bearing supports.    -   the bearing supports comprise bearing surfaces consisting of        e.g. Teflon segments, different composite materials, white metal        and/or steel etc.    -   the adjusting devices are made of hydraulic or pneumatic        cylinders that have a joint of pipe for compressed air and        hydraulic oil respectively through one of the lateral surfaces        of the bearing housing.    -   the maximally and minimally admissible play of the blades is        determinated according to real or expected axial and/or radial        changes of position of the impeller blades.

ADVANTAGES OF THE INVENTION

During normal operation, that is, in open sea, with small risk ofstrange objects being sucked in through the pump and during normalweather conditions, the play between the blades of the impeller and theimpeller chamber may be set to a value that is very favourable for theship's propulsion. If the conditions of operation would become worse theplay can be increased temporarily, considerably reducing or fullyeliminating the risk of unwanted wear etc.

DESCRIPTION OF DRAWINGS

The invention will be explained more in detail with reference to theattached drawings, wherein:

FIG. 1. is a schematic longitudinal section through parts of a jetpropulsion unit for propulsion of a ship according to the presentinvention, the jet propulsion unit comprising an impeller moused at theend of an impeller shaft within an impeller chamber.

FIG. 2 is a schematic longitudinal section through parts of a thrustbearing according to a first embodiment, the thrust bearing beingarranged at the impeller shaft according to FIG. 1 and by means of whichan axial movement of the impeller relative the impeller chamber can beaccomplished.

FIG. 3 is a schematic longitudinal section through parts of a thrustbearing according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1 it is shown schematically and as a longitudinalsection parts of a jet propulsion unit 1 for propulsion of a ship. Beingan integrated part of the hull of the ship 2, the jet propulsion unit 1comprises an impeller pump 3 with a pump housing 4 composed of a frontand a rear part and mounted in the stem part 5 of the hull of the ship 2and usually at or alongside the transom stern 6 of the hull 2.Furthermore, the impeller pump 3 comprises an impeller 7 rotatablyjournalled at a hub housing 8 centrally and stationary mounted within animpeller chamber 9 within the impeller housing 4.

The impeller housing 4 and its impeller chamber 9 forms a tube section10 that is expanded, conical and spherical and attached to a flow tube11 for sea water. The flow tube 11 extends normally (not shown) in acurve obliquely upwardly from a water-inlet that is arranged beneath theload line of the ship and preferably but not necessarily at the bottomof the ship, further to the inlet opening 12 of the impeller chamber 9and thereafter astern to a water outlet 13 at the transom stern 6.During operation of the impeller pump 3 a very forceful water jet S iscreated projecting through an outlet nozzle 17.

The hub housing 8 is firmly attached to the pump housing 4 by means ofseveral guide rails 14 extending between the inside 15 of the pumphousing 4 and the cone-shaped exterior 16 of the hub 8. Said cone 16 ispointing astern and forms together with the inside 15 of the pumphousing 4 the outlet opening, i. e. the outlet-nozzle 17, of theimpeller chamber 9.

Inside the cone-shaped hub 8 there is a bearing seat 18 firmly attachedto the hub 8 via radial rods 19 and arranged to carry a bearingarrangement 20 that may comprise slide bearings or rolling bearings,including combinations of different types of bearings, e.g. spherical,conical or cylindrical thrust bearings and radial bearings. The hub 8can be filled with a lubricant such as oil, thereby being sealed frompenetrating water.

At the embodiment shown the impeller 7 is attached by a journal 21 to animpeller shaft 22, preferably at the end of the journal 21.Alternatively, the impeller 7 may be attached directly to the impellershaft 22. The impeller shaft 22 extends inwardly through the wall of theflow tube 11 at said curve and her into the impeller chamber 9. Theimpeller shaft 22 constitutes either the outer end of the drive shaft ofthe ship or is indirectly connected to the drive shaft. As describedabove, the impeller shaft 22 or the journal 21 are rotatably journalledwithin the hub 8 of the pump housing 4 by means of said bearingarrangement 20.

The impeller 7 comprises a frustum of a cone 23 facing and attached tothe impeller shaft 22 or its journal 21 in such a way that the cone part23 follows the rotation of the impeller shaft 22. Thus, the stationaryhub cone 16 and the impeller cone 23 that is rotatably arranged at thehub cone 16, form together a dome-shaped sphere, around which the pumphousing 4 is shaped to form the likewise spherical impeller chamber 9.Several impeller blades 24 are arranged at the exterior of the frustumof a cone 23 for generating the water jet S that is directed astern.Each impeller blade 24 has such a position and such an extension,comprising length and angle, relative the impeller cone 23 that thedistance, below named the play of the blades 25 (or play 25), betweenthe outer periphery of each blade 24 and the inside 15 of the pumphousing 4 normally becomes very small relative to the diameter of theimpeller 7. For example, during normal operation, an impeller 7 with thediameter of 4.5 meters, may have a play 25 about 0.1% of the diameter ofthe flow tube 11.

At the impeller pump 3 shown in FIG. 1 the impeller 7 is mounted on thejournal 21 of the impeller shaft 22 and the journal 21 is journalled inbearings in the hub 8 by means of the bearing arrangement 20schematically shown. In a preferred embodiment, the bearing arrangement20 comprises one or several slide bearings 26 mounted along the journal21 and/or the impeller shaft 22. Suitably, the slide bearings 26included consist of continuous radial bearings. The bearing arrangement20 is carried by said radial rods 19 fixed within the hub cone 16.

Via the impeller 7, the water jet S directed astern produces a force ofreaction F in the opposite direction that is transferred to the impellershaft 22 possibly via the journal 21. This axial force of reaction Fmust be transferred to the hull of the ship 2, which is done by means ofthrust bearings 27 in a way more closely described below. Only somelimited parts of the shaft 22 are designed to be able to carry theenormous forces of pressure from the impeller 7 and this is done withthe aid of one or several thrust bearings 27 located at a suitabledistance from the impeller 7 itself.

Thus, during normal operation all of the axial forces of reaction F ofthe impeller shaft 22 including all other normally occurring axialforces and/or movements are carried by said thrust bearings 27. The play25 between the impeller blades 24 and the inside 15 of the pump housing4 remains almost constant, at least, all occurring movements stay withinthe limits of the admissible play 25. However, the play 25 might, asdescribed above, be influenced by additional movements or externalforces, that are not to be regarded as normally existing during normaloperation, such as oscillations and vibrations caused by for exampledeformations of the hull of the ship 2 and the impeller shaft 22, toughsea conditions, foreign objects passing through the impeller chamber 9and/or radial changes of position caused by defective mounting of thejet propulsion unit 1 and its propulsion engine. Said oscillations,movements and vibrations might result in such axial and/or radialchanges of position of the impeller blades 24 that they run the risk ofcontacting the inside 15 of the pump housing 4.

For this reason, either all of the thrust bearings 27 are arrangedaxially movable, according to a first design of the present invention,see FIG. 2, or certain parts of the thrust bearing 27 are arrangedaxially movable, a distance sufficient making this risk negligible orfully eliminated, according to a second design of the invention, seeFIG. 3.

The thrust bearing 27 comprises a stabile bearing housing 28 surroundingand enclosing a bearing disc 29 that is preferably a radially protrudingand circular part of the impeller shaft 22, the journal 21, or anintermediate shaft 30 especially adapted to the thrust bearing 27. Thebearing housing 28 has a first and a second radial lateral surface 31,32 arranged astern and afore and at the inside of each lateral surface31, 32 one or several bearing supports 33, 34 are mounted.Alternatively, said bearing supports 33, 34 are arranged, also orinstead, on one or both lateral surfaces 35, 36 of the circular bearingdisc 29.

Also, the bearing housing 28 comprises two sealing flanges 37, 38arranged at each side of the bearing housing 28 and journalled inbearings on the shaft 21, 22, 30 extending through the bearing housing28 and rotating during drive, by means of a separate bearing arrangement39, suitably comprising one or several radial bearings. The bearingsupports (33,34) comprise bearing surfaces (40) consisting of forexample Teflon segments, different composite materials, white metaland/or steel etc.

In the first embodiment (FIG. 2), the entire thrust bearing 27 isarranged to be axially movable along several rods 42 via several guides41 arranged at the bearing housing 28. The rods 42 are arranged at acertain distance from and parallel with the shaft 21, 22, 30 extendingthrough the bearing housing 28 and they are attached to the hull of theship 2 in a suitable way.

In front of the bearing housing 28, i. e. afore, there are severaladjusting devices 43 attached at the hull 2 to achieve the axialmovement astern of the shaft 21, 22, 30, and by that also of theimpeller blades 24, a distance sufficient that the play 25 will be largeenough that the risk that undesired oscillations, movements and/orvibrations will result in such axial and/or radial changes of positionof the impeller blades 24 that they run the risk of contacting theinside 15 of the pump housing 4 will be negligible or fully eliminated.

Preferably, each adjusting device 43 comprises at least one hydraulic orpneumatic cylinder 44 but some of, or all the adjusting devices 43 mayalso be mechanical adjusting devices comprising one or several electricengines (not shown).

Unlike in the first embodiment, the bearing housing 28 in the secondembodiment (FIG. 3) is fixed to the hull 2 in a suitable way (notshown). Here, the adjusting devices 43 are mounted at the inside of thebearing housing 28, more exactly, between the inside of the aforearranged radial lateral surface 32 of the bearing housing 28 and thebearing disc 29. In the embodiment shown in FIG. 3 the adjusting devices43 are hydraulic or pneumatic cylinders 44 that have a joint of pipe 45for compressed air and hydraulic oil respectively through said lateralsurface 32. However, one, several or all of the adjusting devices 43shown may instead be constituted by the above-mentioned mechanicaladjusting devices. Otherwise, the movement of the bearing disc 29, theshaft 21, 22, 30, the impeller 7, the impeller blades 24 and by that thechange of the play 25 is achieved in the same way as in the above firstembodiment.

It is understood that when a smaller play 25 is once more desirable areduced pressure in the cylinders 44 will result in the force ofreaction from the impeller pump 3 forcing the shaft 21, 22, 30 and bythat the bearing disc 29 in the thrust bearing 27 afore. However, whenmechanical adjusting devices 43 are being used, the change is done, inthe same way as the increase of the play 25, but in the oppositedirection. It is understood that in the case the impeller drive is notactivated for achieving the movement afore, additional adjusting devices43 may instead be mounted astern of the bearing housing 28 for achievingthe same function as at the aforedescribed adjusting devices 43, but inthe opposite direction.

Description of Function

According to the invention, the function and the use of the axiallyadjustable or movable thrust bearing 27 for the jet propulsion unit 1 isas follows:

During normal operation, the force of reaction of the impeller 7 iscarried by the thrust bearing 27 at the setting of the play 25 that issuitable for normal operation. The adjusting devices 43 will beactivated either automatically or manually when there is an increasingrisk of damages due to a play 25 that is too small in view of thepresent or expected oscillations, vibrations, movements or conditions,or if such movements etc. are detected. The adjusting devices 43 moveeither the entire bearing housing 28 including the bearing disc 29, orthe bearing disc 29 only, and thereby the shaft 21, 22, 30 astern, whichin its turn increases the play 25 between the impeller blades 24 and theinside 15 of the pump housing 4 due to the spherically shaped and asternwidening impeller chamber 9.

Alternative Embodiments

Thus, the invention is in no way limited to the specifically disclosedembodiments, but every other configuration according to theaforedescribed comes within the inventive concept.

For example, it is understood that instead of a conventional,homogeneous steel shaft the impeller shaft 22 may fully or partlycomprise composite material such as glass fibre, coal fibre, plasticsetc. Furthermore, it is understood that the other parts of the jetpropulsion unit 1, may be made of composite- or light material, such ascoal fibre etc. when it is considered suitable. Bearing surfaces andsurfaces exposed to erosion are suitably provided with preventivecoating, for example by polyurethane. Naturally, combinations of abovementioned materials or of other materials, here not mentioned but withsimilar characteristics, may occur.

Also, it is understood that when some kind of cylinder arrangement forthe movement is used, the design gains a certain dampening function. Theabove-described bearings comprised by the bearing arrangement 20, 39 maybe either water- or oil lubricated.

Finally, it is understood that also the number, dimensions, material andshape of the elements and details comprised by the jet propulsion unit 1must be adapted according to the ship and the other prevailing demandsand conditions. This also applies to the number of jet unit propulsions1 mounted at the ship in question.

1. Jet propulsion unit (1) for propulsion of a waterborne craftcomprising at least one impeller pump (3) with a pump housing (4) beingan integrated part of the hull in which an impeller shaft (22) or ajournal (21) connected to said impeller shaft (22) is rotatablyjournalled with respect to a bearing arrangement, and an impeller (7),having impeller blades (24), the impeller (7) being fixed to saidimpeller shaft (22) or journal (21) so as to follow the rotation of theimpeller shaft (22), the impeller blades (24) being arranged at theimpeller (7) with a play (25) between each impeller blade (24) and theinside (15) of the pump housing (4), characterized in that the jetpropulsion unit (1) also comprises an axially adjustable thrust bearing(27) separate from the bearing arrangement and arranged to cooperate witthe impeller shaft (22) for adjusting said play (25), and that thethrust bearing (27) comprises a bearing housing (28) surrounding andencasing a bearing disc (29) connected to or cooperating with theimpeller shaft (22), further characterized in that said bearing housing(28) is arranged to be axially movable by means of adjusting devices(43) a distance sufficient for the play (25) between the impeller blades(24) and the inside (15) of the pump housing (4) to stay within thelimits of a maximally and minimally admissible play of the blades (25).2. Jet propulsion unit according to claim 1, characterized in that rods(42) are attached at a distance from and parallel with the shaft (21,22, 30) extending through the bearing housing (28) and that the bearinghousing (28) is axially movable along the rods (42) by means of guides(41).
 3. Jet propulsion unit according to claim 1, characterized in thatthe thrust bearing (27) comprises a bearing housing (28) fixed relativeto the hull of the ship (2) and surrounding and encasing a bearing disc(29) connected to or cooperating with the impeller shaft (22), thebearing disc (29) being arranged to be axially adjustable by means ofadjusting devices (43) a distance sufficient for the play (25) betweenthe impeller blades (24) and the inside (15) of the pump housing (4) tostay within the limits of a maximally and minimally admissible play ofthe blades (25).
 4. Jet propulsion unit according to claim 3,characterized in that the adjusting devices (43) are mounted inside thebearing housing (28).
 5. Jet propulsion unit according to claim 1,characterized in that the adjusting devices (43) comprise at least onehydraulic or pneumatic cylinder (44).
 6. Jet propulsion unit accordingto claim 5, characterized in that some of or all the adjusting devices(43) are made of mechanical adjusting devices.
 7. Jet propulsion unitaccording to claim 1, characterized in that the bearing disc (29) is aradially protruding part of the impeller shaft (22), the journal (21),or an intermediate shaft (30) especially adapted for the thrust bearing(27) and connected to the impeller shaft (22).
 8. Jet propulsion unitaccording to claim 1, characterized in that the bearing housing (28)comprises one or several bearing supports (33, 34).
 9. Jet propulsionunit according to claim 9, characterized in that the bearing supports(33, 34) comprise bearing surfaces consisting of e.g. Teflon segments,different composite materials, white metal and/or steel etc.
 10. Jetpropulsion unit according to claim 4, characterized in that theadjusting devices (43) are hydraulic or pneumatic cylinders (44) thathave a joint of pipe (45) for compressed air and hydraulic oilrespectively through one of the lateral surfaces (31, 32) of the bearinghousing (28).
 11. Jet propulsion unit according to claim 1,characterized in that the maximally and minimally admissible play of theblades (25) is determined according to real or expected axial and/orradial changes of position of the impeller blades (24).
 12. A jetpropulsion unit for propulsion of a waterborne craft, comprising: atleast one impeller pump; a pump housing; an impeller shaft rotatablyjournalled with respect to a bearing arrangement; an impeller comprisingimpeller blades; an axially adjustable thrust bearing separate from thebearing arrangement and housed in a bearing housing, and bearing housingadjusting devices for axially moving the bearing housing a distancesufficient to adjust the play to stay within the limits of a maximallyand minimally admissible play of the impeller blades, the impeller fixedto the impeller shaft, or fixed to a journal connected to the impellershaft, to follow rotation of the impeller shaft, the impeller bladesarranged at the impeller with play between each impeller blade and aninner portion of the pump housing, the axially adjustable thrust bearingarranged to cooperate with the impeller shaft for adjusting the play.13. The jet propulsion unit of claim 12, wherein the bearing housingadjusting device comprises rods attached at a distance from and parallelwith the shaft and extending through the bearing housing such that thebearing housing is axially movable along the rods by means of guides.14. The jet propulsion unit of claim 12, further comprising a bearingdisc connected to or cooperating with the impeller shaft, wherein thethrust bearing housing is fixed relative to the hull of the ship andsurrounds and encases the bearing disc.
 15. The jet propulsion unit ofclaim 14, further comprising adjusting devices for axially adjusting thebearing disc a distance sufficient for the play to stay within limits ofa maximally and minimally admissible play of the blades.
 16. The jetpropulsion unit of claim 15, wherein the adjusting devices are mountedinside the bearing housing.
 17. The jet propulsion unit of claim 12,wherein the adjusting devices comprise at least one hydraulic orpneumatic cylinder.
 18. The jet propulsion unit of claim 17, wherein oneor more of the adjusting devices comprise mechanical adjusting devices.19. The jet propulsion unit of claim 12, wherein the bearing disccomprises a radially protruding portion of at least one of the impellershaft, the journal, and an intermediate shaft adapted for the thrustbearing and connected to the impeller shaft.
 20. The jet propulsion unitof claim 12, wherein the bearing housing comprises one or more bearingsupports.
 21. The jet propulsion unit of claim 20, wherein the one ormore bearing supports comprise bearing surfaces comprised of at leastone of Teflon segments, different composite materials, white metal, andsteel.
 22. The jet propulsion unit of claim 15, wherein the adjustingdevices comprise hydraulic or pneumatic cylinders having a passage forcompressed air or hydraulic oil, respectively, through a lateral surfaceof the bearing housing.
 23. The jet propulsion unit of claim 12, whereinthe maximally and minimally admissible play of the blades is determinedaccording to real or expected axial or radial changes of position of theimpeller blade.